Electrophoresis, in general, relates to the phenomenon which occurs when charged particles suspended in a liquid carrier medium are subjected to an electrical field, and the particles are forced to migrate in one direction or another, depending upon the strength of the electrical field and the mobility of the charged particles. Using the principles of electrophoresis, it is possible to deposit a coating on an article by causing charged particles in a suspension to migrate toward and become electrically attached to the surface of the article.
Electrophoretic deposition of particulate materials to form coatings is currently used in a wide variety of industrial applications, such as in the manufacture of enamelled ironware, in applying paint and rubber coatings to metal and plastic articles, in the formation of dielectric coatings on electrical devices, and in other similar industrial processes. Electrophoretic deposition has many advantages over other conventional methods of applying coatings, such as spraying, dipping, brushing, and the like, in that the coating is deposited more effectively with regard to the full utilization of the material in the suspension, as there is substantially no waste of particulate materials; and, the electrophoretically applied coating is generally more uniform in thickness and in density.
One of the principal industrial uses of electrophoretic deposition is to form glass or porcelain coatings on sheet metal articles, such as sheet steel. The resulting glass or porcelain coating formed on the metal articles can be used to protect the underlying metal from corrosion, or to provide a dielectric coating on the surface of the metal article so that it can be used as a substrate on which to assemble electrical devices. In view of the relative commercial importance of forming the glass or porcelain coatings on sheet metal articles, the further description of the invention will be specifically directed toward this application. However, it should be appreciated that the apparatus of this invention can likewise be used with various shaped articles to provide improved coatings in a similar manner.
In the electrophoretic deposition of glass or porcelain-forming materials on a sheet metal article, the initial step is the preparation of a suspension of finely divided glass or porcelain-forming particles in a liquid medium such as water or alcohol. The sheet metal article to be coated is then positioned in the suspension and connected to a cathodic or anodic voltage source. Electrodes are positioned in the suspension adjacent to, but spaced apart from, the article to be coated and are connected to a source of voltage of the opposite polarity to that supplied to the article. When a voltage is applied across the suspension between the article and the electrodes, the particles of the glass or porcelain-forming frit will migrate toward the article to be coated and adhere to the surface of the sheet metal article because of the differences in the polarity of the particles and the sheet metal article. When the desired amount of glass or porcelain-forming frit has adhered to the surface of the article, the deposition process is discontinued and the coated article is removed from the suspension. Normally, thereafter, the coated article is given an additional treatment, such as being fired in the case of glass or porcelain-forming frits, to improve the adhesion of the coating to the surface of the sheet metal article.
In the electrophoretic deposition of coatings using conventional electrophoretic deposition apparatus, a number of problems which are encountered result in unsatisfactory electrophoretically deposited coatings being obtained. One such problem is the lack of uniformity of the deposited coating. The coating which is deposited should be as uniform in thickness as possible for most applications. This is particularly true when the coated substrate is a metal substrate coated with porcelain which will be used as a substrate to support electronic devices, as it is important to have uniform dielectric protection across the entire surface of the substrate. However, certain problems are encountered with the prior art electrophoretic deposition apparatuses, which lead to variations in the thickness of the coating on the article, particularly at the edges.
It was recognized in the art that the suspension comprised of the particulate material and the liquid medium should be as quiescent as possible in the electrophoretic zone, that is, in the zone between the article and the electrodes of the opposite polarity, in order to obtain as uniform a deposit of the particulate material as possible. However, since the suspension is constantly changing in composition because of the removal of particles from the suspension as they are deposited on the article, and also as a result of some separation of the particles from the suspension, it is impossible to maintain a consistent composition of the suspension without circulating the suspension and reconstituting it as required because of the depletion and settling of the particulate material from the suspension. The required circulation of the suspension through the electrophoretic deposition apparatuses of the prior art has caused considerable problems with regard to the uniformity of the application of the coatings as a result of the flow patterns and the disruption of the surface area of the suspension as a result of the fluid movement of the suspension through the apparatus. The problems caused by the circulation of the suspension through the apparatus are particularly evident in certain of the apparatus disclosed in the prior art at the area immediately adjacent the surface of the suspension in the apparatus, in that at these areas rough uneven coatings are commonly obtained because of the variations and the movement of the suspension in these areas.
The circulating suspension should preferably be introduced into the apparatus close to the article to be coated, in a smooth laminar flow, without causing any substantial surface or internal turbulence within the apparatus.
Accordingly, what would be highly advantageous would be an electrophoretic deposition apparatus which included means for improving the circulation of the suspension through the apparatus so as to obtain a smooth, uniform, laminar flow without causing any significant amount of surface or internal turbulences.